chore: use same rust version as https://github.com/fastly/spidermonkey-wasi-embedding/

They drifted apart because spidermonkey-wasi-embedding does not have a rust-toolchain file, which meant the pre-built spidermonkey-wasi would no longer link due to having conflicting/duplicate symbols.

This patch updates our rust-toolchain to match the version used in spidermonkey-wasi-embedding - I have also added the same file to the spidermonkey-wasi-embedding project to ensure they are on the same version.

Fixes #520

Author: Jake Champion